1. Field of the Invention
This invention relates generally to on-board navigational systems and more particularly to the type of navigational systems which continuously display in a plan view the present position of a navigating vehicle in relation to its surrounding environment, such as a ship making a passage within a channel or a harbor or the like.
2. Prior Art
Position fixing systems are generally well known and include such systems as Loran and Decca as well as radar, Omega, satellite and inertial navigation systems.
The Loran system is particularly useful because of the high inherent accuracy of the system. In Loran, radio pulses are generated at each of three stations, one station acting as the master for timing purposes, and at least two others, called secondaries, that establish differetial time and thereby vessel position. At the receiving location, the master signal is received first, followed in succession by two or more secondary signals. The difference in time of arrival between the master signal and each of the secondary signals determines the position of the vessel. Each pair of time differences give rise to a family of hyperbolic loci having intersections that mark the position of the vessel. This is a well known form of hyperbolic navigation utilizing a network of hyperbolic lines of position wherein each hyperbolic line represents the locus of all points that possess predicted equal differences in time of arrival between master and secondary. The family of loci of position forms what is commonly referred to as a "grid" which can visually be depicted, for example, by being overprinted as a set of hyperbolas on nautical charts. High accuracy of the time difference measurement is further achieved by phase comparison of the RF carrier within the pulse transmission. Such a system is known as the Loran-C system.
Notwithstanding, Loran is subject to certain anomalies in the propagation delay from predicted values which cause shifts in the Loran grid. Such shifts can amount to position errors and uncertainties of the order of a few hundred yards or more, which can vary over a twelve month cycle. While such errors can be tolerated in certain areas such as in open water, this is not the case where it is desirable to use Loran for navigation within a very narrow channel in a harbor, along a coastline, or within a river.
Accordingly, it is an object of the present invention to provide an improvement in vehicle navigational systems.
It is another object of the present invention to provide an improvement in the accuracy of a navigational system employing a positional fix system including a navigational display.
Still another object of the present invention is to combine two different forms of radio location apparatus and generate therefrom an integrated navigational display.
And yet a further object of the present invention is to combine an on-board differential Loran system with an on-board radar system and generate therefrom an integrated navigational display from an electronic chart stored in memory and generated in accordance with the present vessel position and further superimposing detected radar targets on the chart while suppressing the land mass echoes detected by the radar.